A new generation of multi-focus X-ray tubes are now being designed to address problems in imaging systems which involve rapid movement of the object under inspection. This is particularly important in tomographic imaging systems where object motion can create unacceptably high levels of artefact in reconstructed images. To address this problem, multi-focus X-ray sources are proposed in which often many hundreds of individual electron guns are arranged, typically into a circular array, and each electron gun is switched on sequentially to irradiate a respective point on a circular anode with the same radius as that of the electron guns. This forms a rotating X-ray source without the need for physical motion of the assembly, hence creating the opportunity for very high speed tomographic imaging.
In such tomographic X-ray systems, it is often desirable to provide materials discrimination capability which is typically achieved through the use of the reconstructed grey level of the tomographic image with calibration back to a set of known reference standards (e.g. air, water, aluminum).
It is recognised that further materials discrimination capability can be achieved when the energy spectrum of the X-ray beam is taken into account since each spectral component in the incident X-ray beam is attenuated to a different amount by each component material within the object under inspection. Low atomic number materials provide modest attenuation of low energy X-rays whilst high atomic number materials provide significant attenuation of low energy X-rays. By analysis of the X-ray spectrum after filtering by the object, it is possible to obtain further materials discrimination than if the X-ray spectrum is simply integrated.
In a practical X-ray system, it is expensive to measure the energy of every single X-ray photon that arrives at the detector. This is because the arrival rate of photons at each detector element is relatively high (often over 1 MHz photon arrival rate) and the complexity and associated power dissipation of the detection electronics becomes a significant issue.
One means to simplify the situation is to utilise more than one inexpensive integrating detector per imaging channel, but with a filter placed between one detector and the other. The filtered detector is generally made thick to measure the high energy components of the X-ray beam transmitted through the object. The unfiltered detector is usually quite thin and so responds preferentially to the low energy components of the transmitted X-ray beam.